In order to investigate the effects of ventilation on Opalinus clay in the Mont Terri Underground Rock Laboratory, Switzerland, a Ventilation Experiment (VE) was installed and executed in a non-lined microtunnel of 10 m in length and 1.3 m in diameter, for the period of 2003–2006 (Fig.1) by a EC financed NFPRO project (see Mayor, J.C. and M. Velasco, 2008, The Ventilation Experiment Phase II (Synthesis report), NFPRO Project WP4.3, Deliverable D4.3.18.). The issues of importance that were investigated during the VE include: desaturation/resaturation of the rock, phase change, air/rock interface behaviour, damage/microcracking of the host rock due to hydromechanical and/or chemical effects, and evolution of the Excavation Damaged Zone (EDZ). The VE experiment was taken as the physical basis of Task A of the Decovalex-2011 project, starting in 2008, with the main objective of examining the hydro-mechanical and chemical changes that may occur in argillaceous host rocks, especially in relation to the coupling with the ventilation of drifts, through numerical modelling. The main items of investigation included: identification of relevant processes of Opalinus Clay parameters on the basis of the laboratory drying test; hydromechanical modelling of the VE up to the end of the first drying phase on the basis of the laboratory parameters and calibration of the models; and advanced hydromechanical modelling of the VE, including blind prediction of the second drying period.
The participating teams and the corresponding funding organisations were: Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, China; Commissariat à l’Energie Atomique (CEA), France, sponsored by IRSN, France; Quintessa Ltd., UK, sponsored by RWM, University of Edinburgh, UK, sponsored by RWM; Japan Atomic Energy Agency, Japan.
The modelling approach of Task A was therefore to perform the hydro-mechanical and geochemical modelling of the different phases of the VE tests and to compare calculated results with experimental observations. This allowed checking the capabilities of the various modelling tools and to advance the understanding of ventilation effects on an argillaceous host rock.
Task A was organised in a progressive manner in terms of complexity of the computations to be performed, and geared towards the full HMC understanding of the VE, the final objective of the Task. Five steps were defined, including a preparation step to begin smoothly into the problem:
Step 0: Identification of relevant processes and of Opalinus Clay parameters. Modelling of a laboratory drying test;
Step 1: Simple Hydro-mechanical modelling up to the end of Phase 1;
Step 2: Advanced hydro-mechanical modelling up to the end of Phase 2 using parameters back-calculated from Step 1. Advanced features such as permeability anisotropy, rock damage and permeability increase in the damaged zone possibly considered;
Step 3: Hydro-mechanical and geochemical modelling of the full test. Conservative transport and one species considered; and
Step 4: Hydro-mechanical and geochemical modelling of the full test. Reactive transport and full geochemical model (optional).
Main achievements and outstanding issues
The Task A models led to a satisfactory reproduction of an important quantity of measurements (water mass balance of the experiment, the relative humidity, pore water pressure, water content and deformation). The consistent results between the different codes, and between the codes and the measurements, were an important validation of the numerical tools.The analysis of the laboratory drying test and the in-situ Ventilation Experiment included the identification and discussion of all possible processes that could influence the test results and more particularly the rock behaviour.
The modelling work carried out in Task A allowed for assessing the relevance of a number of desaturation processes, concerning determination of groundwater permeability of the porous rocks, importance of vapour diffusion, dependency of the water permeability on the saturation degree, importance of accurate representation of the retention curve for dealing with partially saturation related issues, anisotropic rock properties (permeability and deformability), hydrological effects on stress redistribution and the EDZ, among others. The main outstanding issue was the inadequate depth of research in chemical coupling and uncertainties of the effects of fracture systems in the host rock for both flow and transport processes, as well as a lack of extended research on the impact of performance and safety assessments over a longer time period.
B. Garitte, A. Bond, A. Millard, C. Zhang, C. Mcdermott, S. Nakama, A. Gens, Analysis of hydro-mechanical processes in a ventilated tunnel in an argillaceous rock on the basis of different modelling approaches, Journal of Rock Mechanics and Geotechnical Engineering, Volume 5, Issue 1, February 2013, Pages 1-17
A. Bond, S. Benbow, J. Wilson, A. Millard, S. Nakama, M. English, C. McDermott, B. Garitte, Reactive and non-reactive transport modelling in partially water saturated argillaceous porous media around the ventilation experiment, Mont Terri Journal of Rock Mechanics and Geotechnical Engineering, Volume 5, Issue 1, February 2013, Pages 44-57
A. Bond, A. Millard, S. Nakama, C. Zhang, B. Garritte, Approaches for representing hydro-mechanical coupling between sub-surface excavations and argillaceous porous media at the ventilation experiment, Mont Terri Journal of Rock Mechanics and Geotechnical Engineering, Volume 5, Issue 2, April 2013, Pages 85-96
A. Millard, A. Bond, S. Nakama, C. Zhang, J. Barnichon, B. Garitte, Accounting for anisotropic effects in the prediction of the hydro-mechanical response of a ventilated tunnel in an argillaceous rock, Journal of Rock Mechanics and Geotechnical Engineering, Volume 5, Issue 2, April 2013, Pages 97-109
C. Zhang, X. Liu, Q. Liu, A thermo-hydro-mechano-chemical formulation for modeling water transport around a ventilated tunnel in an argillaceous rock, Journal of Rock Mechanics and Geotechnical Engineering, Volume 5, Issue 2, April 2013, Pages 145-155.